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Most of us have a reasonable idea of what the near-death experience (NDE) is. It's said to happen when you are in the actual process of dying, and you hear strange noises, then feelings of blissful peace and joy sweep over you.

You then float out of your body, seeing it from above, and head towards a tunnel and you enter the tunnel. At the far end of the tunnel is a bright light, but as you get close to the light, you are met by someone, already dead, who tells you that this is not yet your time, and regretfully, you are plunged back to our prosaic planet with other Earthlings.

In a nutshell, the common beliefs are that the NDE happens only to those who are dying, and that it is also proof of an afterlife. But neither belief is correct.

In 1926, Sir William Barrett, a fellow of the Royal Society, published a slim volume on deathbed visions. The survivors described to him the basic elements of the NDE. But the NDE became really popular only in 1975, with the publication of Raymond Moody's best-selling book, Life after Life.

This book recounts the anecdotes of some 50 survivors. At first, there was some resistance to the concept of the NDE, but with further research it was found to be surprisingly common.

Some 10 to 20 per cent of survivors of heart attacks report having an NDE. Similar figures come from survivors of electrocution, coma, attempted suicide, near-drowning and massive shock due to blood loss, infection or anaphylaxis.

However, NDEs are not always related to death (despite the 'D' in NDE).

For example, the training of combat fighter pilots can involve high G forces in a centrifuge, which sends the blood away from the brain to the feet, leading to unconsciousness. The typical pathway involves a greying of the vision, followed by total blackout and vivid dreams.

Some 20 per cent of pilots who blackout in this centrifuge training report classic NDEs, with the out-of-body experience. And to further separate the NDE from death, in hospital situations where patients report an NDE, some 50 per cent of them would have survived without any medical intervention.

We still don't know exactly what causes an NDE, simply because the brain is so fiendishly complicated (as is the concept of consciousness).

The brain receives information from our primary senses, such as vision, sound, taste, smell and touch. Skin sensors add information on temperature, pain and pressure, while sensors in the ear give information on your balance and orientation.

And sensors in the joints, tendons, muscles and bones tell the brain where our limbs are located. All of these bits of information, and many more as well, flow into our brain like many rivers into a sea, to give us that strange perception that we call 'consciousness'.

There are dozens of theories on what causes an NDE. They include bizarre electrical activity in parts of the brain, endorphins in the brain stem, strange natural chemicals in the brain such as endopsychosin, intrusion of our normal REM dream sleep into our consciousness, and many other theories.

These theories all have some elements of fact to back them up, and probably, they all are involved in varying amounts, depending on the person.

But recently, Dr Olaf Blanke, a neurologist at the Ecole Polytechnique Federale de Lausanne in Switzerland, was able to consistently duplicate some of the elements of the NDE in the lab.

He stimulated, with electricity, a part of the brain called the left angular gyrus. Patients immediately reported the impression of a shadowy person lurking behind them. And when he stimulated the right angular gyrus, they reported that they left their physical body and could look down on it from above — the classic out-of-body experience.

At some stage in the future, we might know enough about the brain to fully understand the NDE. But right now, we know that you don't have to be dying to have one, and that it neither proves nor disproves the existence of an afterlife.